In the medical field, portable radiographic image capturing apparatus such as an FPD (Flat Panel Detector) have been used for detecting the intensity of radiation that has passed through a human body in order to capture an image of the inside of the human body. An FPD (hereinafter referred to as an “electronic cassette”) can be used flexibly on patients who cannot move, because the electronic cassette is capable of capturing images of a patient lying on a bed or the like, and can be changed in position in order to adjust the areas to be imaged.
Electronic cassettes include an indirect-conversion-type electronic cassette having a scintillator for temporarily converting radiation into visible light, and a solid-state detector for converting visible light into electric signals. In particular, an electronic cassette including a scintillator made of CsI (cesium iodide) has a high response speed and a high detection capability, and hence is of high performance.
However, an electronic cassette having a scintillator made of CsI tends to suffer from a so-called bright-burn phenomenon, which is a type of afterimage, as a phenomenon unique to CsI scintillators. Bright-burn phenomena occur especially if the scintillator is irradiated with intense radiation. According to a radiographic image capturing process, the electronic cassette captures an image with radiation at an increased dose, and thereafter, an image is captured again with radiation. If the electronic cassette captures an image with radiation at an increased dose, many traps are developed unevenly in the scintillator. If an image is captured again with radiation using the electronic cassette, information represented by the traps is added as radiographic image information and is output from the scintillator. The scintillator tends to bring about irregular sensitivity rises due to bright-burn phenomena, which result in a reduction in contrast and hence a drop in image quality. These problems lead to a reduction in accuracy if subjects are diagnosed by interpreting the captured image.
Heretofore, methods have been proposed for minimizing bright-burn phenomena, as disclosed in Japanese Laid-Open Patent Publication No. 2003-107163, Japanese Laid-Open Patent Publication No. 2010-523997, and Japanese Laid-Open Patent Publication No. 2009-514636.
According to Japanese Laid-Open Patent Publication No. 2003-107163, the scintillator is heated to discharge electric charges held by deep traps.
According to Japanese Laid-Open Patent Publication No. 2010-523997, after a radiographic image has been captured, ultraviolet radiation is applied to the scintillator from a side opposite to an X-ray-irradiated surface thereof, thereby causing the scintillator to emit light, and image information generated by the emitted light is used to perform a correction (calibration).
According to Japanese Laid-Open Patent Publication No. 2009-514636, a main image capturing process is preceded by application of radiation to the scintillator in order to form deep traps in the scintillator over the entirety thereof, thereby holding local sensitivity rises to a minimum.
The bright-burn phenomenon is generally referred to as an afterimage phenomenon. Afterimage phenomena also occur in direct-conversion-type electronic cassettes made of selenium, and are referred to as “ghost” phenomena. Similar to the case of bright-burn phenomena, ghost phenomena occur due to electric charges, which remain in selenium from a preceding image capturing process, and are added and output as radiographic image information in a subsequent image capturing process. Thus, the scintillator tends to bring about irregular sensitivity rises due to such ghost phenomena, which leads to a reduction in contrast and hence a drop in image quality.
Heretofore, an attempt has been made to reduce the occurrence of ghost phenomena with an upper electrode, which is disposed directly in physical and electric contact with an electric charge generator layer that includes a base made of amorphous selenium (see Japanese Laid-Open Patent Publication No. 2006-263452). According to another prior art example, an upper electrode is disposed over an electric charge generator layer, which includes a base made of amorphous selenium with a non-insulating organic layer interposed therebetween, thus making it possible to transport electric charges across the non-insulating organic layer in order to reduce the occurrence of ghost phenomena (see Japanese Laid-Open Patent Publication No. 2007-199065 and Japanese Laid-Open Patent Publication No. 2007-296337). Since there is no electric charge barrier layer, thin-film transistors, which are coupled with signal storage capacitors, are likely to experience break down upon exposure to intensive radiation. However, a structure for positively passing a leakage current is employed in order to prevent the thin-film transistors from breaking down.